The objective of this search is to elucidate the mechanisms involved in hemoglobin (Hb) switching and regulation of erythropoiesis in amphibians going through metamorphosis. Anurans (frogs and toads) undergo a switch from larval Hb to adult Hb synthesis during metamorphosis that is analogous to the switch from to adult Hb in humans at about the time of birth. The two species of anurans chosen for this investigation have particular advantages for studying the mechanisms of Hb switching and for investigating ways of potentiating synthesis of either immature (larval) or adult Hb's. The bullfrog Rana catesbeiana is advantageous because larval Hb and adult Hb are found in separate red blood cells (RBC's) during metamorphosis. Thus, it should be easier to differentiate between factors which regulate larval versus adult Hb synthesis than it would be in an animal in which both Hb types coexist in each RBC. The South African clawed toad Xenopus laevis is a species in which both larval and adult Hb's are found in the same RBC during metamorphosis. It will be informative to see if factors which promote synthesis of specific types of Hb's in Rana catesbeiana will work the same way in this closely related species. Hemopoietic cell grafts will be used to attempt to answer the question of whether or not larval and adult RBC's share common stem or progenitor cells. Both in vivo and in vitro (tissue culture) approaches will be used to investigate the effect of erythropoietic tissue microenvironments, hormones, and anemia-producing agents on specific Hb synthesis as evaluated by gel electrophoresis and incorporation of radioactive compounds into Hb.